This invention relates generally to optical telecommunications networks, and in particular to methods and apparatus for installing guide tubes through which fiber optic cables are to be routed within a protective conduit such as an underground duct. Specifically, this invention provides an improved bundle of guide tubes containing a filling body or spacer that facilitates installation through the bends and undulations of a duct trajectory while providing mechanical protection against excessive forces that may be applied to the guide tubes after installation.
Various factors should be considered when a fiber optic cable is installed in a protective duct. A major concern is avoidance of damage to the cable during installation. Another concern is ease of installation and the desire for a reduction in the amount of time needed to install the cable. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the cable run.
Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose two or more guide tubes, whose interiors may have a lower coefficient of friction than the protective duct, are installed in the existing protective duct, thereby establishing separate channels or sub-ducts in which one or more cables, optionally at a later time, can be blown or flowed through the protective duct over a greater length. It may also be desirable to install in an existing protective duct a larger number of guide tubes with a smaller cross section than that of the existing protective duct if it is desired to use each of the smaller tubes as a separate channel or subduct for single-core or multi-core copper or glass fiber cables. Further, it may be necessary to install in an existing duct a protective guide tube with a water barrier, so that in the existing duct, whose interior gradually fills up with water through diffusion, a waterproof conduit is created by means of the protective guide tube, this waterproof conduit allowing the routing of cables without a water shield.
An early approach to duct channelization is described in EP-A-0108590 to Reeve in which a ducting network, the ducts of which have previously been provided with a number of separate channels, allows a separate lightweight and flexible fiber optic member to be blown in by compressed air in each channel without armor or water barrier. The duct provided with channels protects the cables against external influences, such as moisture and the like. In this way, a network with individual fiber optic members to each customer is created, with the fiber optic members being arranged in parallel channels up to the branches. U.S. Pat. No. 5,884,384 to Griffloen describes combining high-speed airflow with a pushing force to install channelization guide tubes in an existing protective duct. In the air blowing/pushing technique the air-drag propelling forces on the bundle are distributed over the entire length of the guide tubes. The longitudinal forces imposed on the guide tubes are kept low and because of that friction arising along curves of the duct trajectory is minimized.
During blowing/pushing installation of guide tubes, the propelling air-drag force developed by the volumetric flow of air through the protective duct is proportional to the compressor output pressure and bundle diameter. However, the frictional load imposed by rubbing engagement of the guide tubes against the duct is proportional to the bundle weight, hence to the square of the bundle diameter. Moreover, a bundle that fills the duct for a large part, when the bundle is tight, is subjected to extra friction caused by bends and undulations in the duct trajectory due to the stiffness of the bundle, which increases with the fourth power of the bundle diameter. On the other hand a bundle that just fits in the protective duct can be pushed harder without buckling, but the frictional loading caused by rubbing engagement imposes a limit on the continuous installation length that can be obtained by pushing/blowing for such large bundle diameters.
For making the most productive use of available underground duct space, it has been conventional practice during the initial installation to fill the protective duct as completely as possible with channelization guide tubes of various diameters to accommodate present and anticipated cable branching/drop requirements. In previous guide tube installations, the size and number of guide tubes have been selected to provide a high filling of the protective duct. However, it was found in practice that such jobs incur increased installation time, along with a reduction of the overall bundle length that can be blown in continuously, thus requiring more guide tube joints, more duct junctions and, last but not least, a shorter maximal distance between handholes or manholes (if installation is done in an existing duct trajectory, where digging the street again is to be avoided).
For a loose bundle of guide tubes an intermediate filling degree is desirable to allow the guide tubes to move away when the duct is indented, thus providing some mechanical protection. It has been demonstrated that guide tube bundles with a cross-sectional area of approximately half the inner cross-sectional area of the protective duct are protected just as good as armored cables.
It can be understood that a bundle with a larger diameter is more difficult to blow in. The friction forces that oppose installation are proportional to bundle weight, hence to the square of bundle diameter. The air-drag that assists installation is proportional to bundle diameter. But, for a loose bundle that fills the protective duct for a large part the negative effect of extra friction in bends and undulations of the duct trajectory due to the bundle stiffness is, surprisingly enough, not as severe as for a tight bundle, where the tubes cannot slide freely and where the stiffness of the bundle increases with the fourth power of bundle diameter. In order to maintain a small stiffness of the loose bundle it is required that the guide tubes can slide over each other without too much friction. Still the positive effect, that a bundle that just fits in the duct can be pushed harder without buckling, exists.
That installation of a loose bundle of tubes is more troublesome than expected was made evident during a test installation in which a loose bundle of ten guide tubes (7/5.5 mm) were blown into a 40/33 mm protective duct. This provided a filling factor of about 50% of the duct cross-sectional area. This bundle was more difficult to blow in (1200 m reached) than a bundle of seven guide tubes (1500 m reached without problems). When the same installations were repeated in a duct trajectory with many bends the difference in performance became even larger. The performance of installation of bundles with higher filling factor drops even more rapidly. These drops in installation length were not predicted by theory (computer simulation).
When the filling degree of the bundle is low (for example 50% as shown in FIG. 2) there is space enough for crossing of guide tubes to occur. When installation is done by pushing, the guide tubes may buckle and cause extra friction, see FIG. 1. Here a longitudinal pushing force FL results in a transversal force FT between the guide tube 12 and the protective duct 14. When the pushing forces are taken away the friction caused by the buckling will disappear again. The guide tube 12 when forming a 3-dimensional restriction appears as two tubes (indicating the end positions in the S-shape) and a bend loop in between them (indicating the S-shape between the end positions), and rubs in contact with the inner duct sidewall 20 on both sides because that is what happens when the permanent restriction is generating friction.
As the bundle more completely fills the duct more crossing may cause the guide tubes to be pressed against the duct sidewall. Also, a three-dimensional restriction in the form of a permanent “knot” or tangle may be formed. Because the propelling forces are low during pushing/blowing installation, these knots can have an adverse effect on installation performance. Also, buckling can cause permanent friction now, as a result of the three-dimensional restriction in the duct space 20, see FIG. 2. A guide tube can bend in the plane perpendicular to the buckling plane and form a permanent “spring” that remains when the pushing forces are taken away.